1. Field of the Invention
The present invention relates to the treatment of wastewater or sewage to remove impurities therefrom.
2. Description of the Prior Art
There are a large number of industries which, because of the nature of the product thereof, produce a wastewater which is very high in organic constituents and is generally not acceptable for disposition in a lake, a river or the like because of the high oxygen demand associated therewith. In particular, environmental laws and regulations are becoming increasingly stringent in requiring the removal of chemical and biological oxygen demanding substances (COD and BOD) from wastewater before such water is allowed to flow into public water ways. An example of such a wastewater is the effluent from a food processing plant. In such a plant, water is used in the preparation or processing of fresh vegetables and other manufactured food items, as well as for steam generation and sanitary facilities. Before the water used in such processing may be discharged, the waste deposited during processing must be removed to a degree that will satisfy the various, imposed environmental laws and regulations.
The prior art has suggested a number of devices and processes for removing impurities or waste from the wastewater so as to improve the quality thereof. Such devices have included reactors and clarifiers, and such processes have included both aerobic and anaerobic processes.
In a typical aerobic process, the wastewater is directed through a reactor vessel with microorganisms therein and is thereafter passed into a settling basin. An underflow from the settling basin is then returned to the reactor vessel. In the course of such process, the wastewater is mixed with large quantities of organisms and air. Microorganisms use the organic waste for food, and use the oxygen in the air to burn a portion of this food to produce carbon dioxide and water for energy. Since these organisms obtain much energy from the oxidation, their growth is rapid and a large portion of the organic waste is converted into new cells. The portion converted to cells is not actually stabilized, but is simply changed in form. Although the cells can be removed from the waste stream, the biological sludge they produce still presents a significant disposal problem.
In the anaerobic process, wastewater is injected into a reactor vessel and contacted with sludge containing microorganisms. Effluent leaving the reactor vessel is passed through a degassifier and into a settling tank. Heavier sludge falls to the bottom of the settling tank and is recirculated to the reactor vessel. Waste water is also mixed with large quantities of organisms, but unlike the aerobic process, air is excluded. Under these conditions, bacteria grow which are capable of converting the organic waste to carbon dioxide and methane gas. Unlike aerobic oxidation, the anaerobic conversion to methane gas yields relatively little energy to the microorganisms. Thus their rate of growth is slow and only a small portion of the waste is converted to new cells, the major portion of the degradable waste berng converted to methane gas. Such conversion to methane gas represents waste stabilization, since this gas is insoluble and escapes from the waste stream where it can be collected and burned. The prior art has recognized that the bi-product of methane gas is a good source of fuel energy and has used same for heating buildings, running engines, and producing electricity.
The anaerobic treatment process does have a number of disadvantages. The major disadvantage is that relatively high temperatures are required for optimum operation; temperatures in the range of 85.degree. to 95.degree. F. are preferred. Another disadvantage of anaerobic treatment is related to the slow rate of growth of the methane producing bacteria. As a result, longer periods of time are required for starting the process. The slow rate of growth also limits the rate at which the process can adjust to changing waste loads, temperatures and other environmental conditions. Further, the gas produced in anaerobic treatment makes the suspended particles buoyant and difficult to settle. Therefore, a degassifier is frequently required between the digester and the settling tank in the anaerobic contact process to permit proper settling of the suspended solids.
U.S. Pat. No. 4,311,593 of Benjes et al. discloses an anaerobrc reactor or biological digester, where effluent in the form of wastewater from the production of alcoholic beverages is introduced at the top of the reactor and, then, flows downward through a media. Sloughed biomass along with a portion of the wastewater is collected at the floor of the digester and is recirculated by a pump to be reintroduced at the top of the reactor to engage and interact with the digester influent. Methane is collected at the top of the digester and is conducted to a combuster, where the methane is burned to produce heat for the distilling the fermented molasses. Further, effluent may be removed from the digester at its lower end and forced by a pump to a waste disposal.
U.S. Pat. No. 2,360,811 of Kelly et al. discloses a purification system where wastewater is successfully processed by a screening station, a grit washing station, a gas diffusing station, a vacuum flotation apparatus, wherein settling of large solid particles occurs and, finally, a biological digester. A burnable gas is collected at the top of the digester and is conducted to an incinerator, where sludge is heated to form sludge cakes. Digested sludge is removed from the bottom of the biological digester and is directed through a dewatering station for sludge filtering. Filtrate from the dewatering station and supernatant from the digester are reinserted into the process ahead of the gas diffusing station. The wastewater is introduced into the biological digester and is directed downward through its media.
U.S. Pat. No. 3,623,976 of Cessna discloses the recirculation of sludge collected in a bottom portion of an activated biological filter. In a first recirculation path, effluent is taken from the bottom of the biological filter and is passed through a clarifier before being pumped into a top portion of the biological filter. In a second path, effluent is taken directly from the bottom portion of the filter and is recirculated to the top thereof. Alternatively, Cessna permits sludge to be taken from the clarifier and discharged as waste.
U.S. Pat. No. 4,366,059 of Witt et al. discloses an anaerobic treatment plant including a biological filter. Particles of solid biomass are withdrawn from the filter and recycled to the base of the filter together with fresh acidic waste and added inorganic alkaline material. Witt et al. suggests that the alkaline material may be magnesium oxide or sodium formate.